Is the iPhone SE better than iPhone 6 or 6s?

All depends what you want... They all share the same majority parts but The iPhone 6s+ is the iPhone with all the bells and whistles on it, the iPhone 6s (which is what I have) is about 95% the same as the 6s+ ..... Just missing a few things. The iPhone SE is missing a few more items further! SE has no force touch, a very slightly less capable camera set up. All depends what's important to you. All are very good phones. Try the apple site to compare the models in detail. The SE is a 4" so the screen size might be what makes your mind up...

There is no "better" in this comparison. The SE has a chipset which should be more capable than the original iPhone 5/6 chipsets and will probably be supported for longer than the older models (6S will probably have similar support). That could be considered to be better than iPhone 6 and same as 6S. The size of the phone and screen are different, that could be an advantage or a disadvantage depending on how each user considers needs and priorities. The SE is more of an "entry level" specification (at least on pricing), which could be considered as a disadvantage when it comes to trading for upgrade or selling it on to a new owner.

Basically, anyone thinking about buying a new and unfamiliar smartphone is recommended to get some hands-on time in a local phone store (or borrow one from a friend for half an hour or so) to see how it feels in the hand. If it doesn't fit your paws or you can't read the screen easily, don't buy that model. If it feels good it stays on the list of contenders, give it points out of 10 and when all contenders have been considered review your ratings.

6s is definetely better than SE in term of having 3D touch, better selfie camera
but SE definetely faster than 6s but only if you're a heavy user like me, gaming, working, messaging, emailing, etc
SE is way faster than 6 for sure